1. Field of the Invention
This invention relates to a 2,6-di-tert-butyl-4-substituted thiopyrylium salt, a process for the production of said salt and a photoconductive composition containing said salt.
2. Description of the Prior Art
As is known in the art, thiopyrylium and pyrylium dyes are used in various applications. For example, as disclosed in Japanese Patent Publication No. 40900/71, they are used in a direct positive photographic silver halide emulsion as an electron acceptor. They are also useful as a spectral sensitizer for a photoconductor, especially an organic photoconductor, as taught in Davis et al, U.S. Pat. No. 3,141,700, Van Allan et al, U.S. Pat. No. 3,250,615 and Reynolds et al, U.S. Pat. No. 3,938,994.
Photoconductors sensitized with thiopyrylium and pyrylium dyes are used in the various applications as disclosed in the above-described patents, and they are particularly important for use in electrophotography such as xerography or electrofax.
Hitherto known thiopyrylium dyes, however, suffer from the disadvantage that they have a plurality of absorption bands in the visible region. In particular, almost all of the dyes exhibit absorption in the blue region. This means that the dyes spectral sensitize in a plurality of wave length regions. Therefore, in forming a color image by use of such thiopyrylium dyes as a sensitizer for photoconductive particles, for example, in obtaining a color image from a mixture of three colored particles according to the photoelectrophoretic electrophotography, serious problems occur.
The principle of the photoelectrophoretic electrophotography is detailed in U.S. Pat. No. 3,384,448, according to which a suspension of photoconductive photosensitive particles in an insulating liquid is placed between a pair of electrodes, at least one of which is transparent to light and across which a differential voltage is applied and the suspension is exposed imagewise through the transparent electrode. The photoconductive photosensitive particles selectively migrate to one electrode to form a visible image on that electrode.
To produce a polychromatic image, a suspension comprising a mixture of cyan colored particles sensitive to red light, magenta colored particles sensitive to green light and yellow colored particles sensitive to blue light is set on the above described system wherein it is exposed imagewise through a multicolor original image, e.g., a color slide (or by the reflective printing method) using white light, and one operation of imagewise exposure produces a subtractive color positive or negative image on the transparent electrode.
Illustrative particles suitable for producing such subtractive color images are described in U.S. Pat. No. 3,384,448, Japanese Patent Publication No. 21781/68 (U.S. Pat. Nos. 3,681,064 and 3,384,556) and Japanese Patent Application (OPI) No. 143822/77 (U.S. Pat. No. 4,032,339) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and they are cyan, magenta and yellow pigments the principal absorption bands of which correspond to their principal sensitive regions.
In addition to these three colored photoconductive pigments, U.S. Pat. No. 3,384,448 teaches electrically photosensitive particles which contain a spectral sensitizer so that they are sensitive to radiation in the visible spectral range.
In obtaining a color image from such a mixture of three colored particles according to the photoelectrophoretic electrophotographic method, the use of known thiopyrylium dyes as a spectral sensitizer for photosensitive particles has resulted in the formation of those images having insufficient color separation because of their spectral sensitization in a plurality of wave length regions.
This indicates that the known thiopyrylium dyes have been unsuitable for use in the production of color images by the photoelectrophoretic electrophotographic method.
The inventors have found novel thiopyrylium dyes which are free from the disadvantages as described above and provide photoconductive substances with higher sensitivity than do conventional thiopyrylium dyes, and a process for the production of said thiopyrylium dyes.